Summary:
Make SnapshotConcurrentAccessTest run in the beginning of the queue.

Test Plan
`make all check -j64` on devserver
Closes https://github.com/facebook/rocksdb/pull/2962

Differential Revision: D5965871

Pulled By: yiwu-arbug

fbshipit-source-id: 8cb5a47c2468be0fbbb929226a143ec5848bfaa9